• Vector data
• represents space using points, lines and polygons (“features”)
• each of these can have additional data (“attributes”)
• R package sf
  
  
• Raster data
• represents space using cells of equal size
• each cell can have additional data (“layers”)
• R package raster

• A feature is a thing… a city, a house, a tree
• has a geometry describing where it is located
• has attributes describing its other properties
  
  
• A feature may be made up of other features
• pixel vs. image

• open standard for spatial data types
  
  
• points, lines and polygons = “geometries”
  
  
• geometries are hierarchical
• set of points is a line
• set of lines forms a polygon
• several polygons is a multipolygon
  
  
• 17 geometries, of which we look at 7
  
  
• “data frames with a spatial extension”

• A point: st_point()
• A linestring: st_linestring()
• A polygon: st_polygon()
• A multipoint: st_multipoint()
• A multilinestring: st_multilinestring()
• A multipolygon: st_multipolygon()
• A geometry collection: st_geometrycollection()
  
  
• Can be created from vectors, matrices, or lists

# POINT
(pt1 <- st_point(c(5, 2)))

# LINESTRING
(ls1 <- st_linestring(rbind(c(5, 2), c(1, 3), c(3, 4))))

# POLYGON
pl <- list(rbind(c(5, 2), c(1, 3), c(3, 4), c(5, 2)))
(poly1 <- st_polygon(pl))

# MULTIPOINT
mpmat <- rbind(c(5, 2), c(1, 3), c(3, 4))
(mp1 <- st_multipoint(mpmat))

# MULTILINESTRING
mll <- list(rbind(c(1, 5), c(4, 4), c(4, 1), c(2, 2)), rbind(c(1, 
    2), c(2, 4)))
(mls1 <- st_multilinestring((mll)))

# MULTIPOLYGON
mpyl <- list(list(rbind(c(1, 5), c(2, 2), c(4, 1), c(1, 5))), 
    list(rbind(c(0, 2), c(1, 2), c(1, 3), c(0, 3), c(0, 2))))
(mpoly1 <- st_multipolygon(mpyl))

# GEOMETRYCOLLECTION
gcl <- list(ls1, pt1, mpoly1)
(geomcol <- st_geometrycollection(gcl))

• Objects we’ve made so far have class sfg

class(pt1)

## [1] "XY"    "POINT" "sfg"

class(mp1)

## [1] "XY"         "MULTIPOINT" "sfg"

• A simple feature geometry column (sfc) is a list of sfg objects
• Important because usually we have more than one feature

# sfc POINT
pt1 <- st_point(c(5, 2))
pt2 <- st_point(c(1, 3))
(points_sfc <- st_sfc(pt1, pt2))

## Geometry set for 2 features 
## geometry type:  POINT
## dimension:      XY
## bbox:           xmin: 1 ymin: 2 xmax: 5 ymax: 3
## epsg (SRID):    NA
## proj4string:    NA

class(points_sfc)

## [1] "sfc_POINT" "sfc"

is.list(points_sfc)  # is this a list?

## [1] TRUE

is.list(mp1)  # is this (multipoint) a list?

## [1] FALSE

st_geometry_type(points_sfc)

## [1] POINT POINT
## 18 Levels: GEOMETRY POINT LINESTRING POLYGON MULTIPOINT
... TRIANGLE

• sfg and sfc only tell you about locations
  
  
• to these we add non-geographic attributes associated with each feature
  
  
• an sf object is a data frame combines these two things:
• attributes are stored in a regular data.frame
• location information (co-ordinates) is stored in one (simple feature geometry) column (sfc) of this data frame
  
  
• Simple features = data frames with spatial attributes stored in a list-column

lnd_point <- st_point(c(0.1, 51.5))                 # sfg object
lnd_geom <- st_sfc(lnd_point, crs = 4326)           # sfc object
lnd_attrib <- data.frame(                           # data.frame object
  name = "London",
  temperature = 25,
  date = as.Date("2017-06-21")
)
lnd_sf <- st_sf(lnd_attrib, geometry = lnd_geom)    # sf object
lnd_sf

## Simple feature collection with 1 feature and 3 fields
## geometry type:  POINT
## dimension:      XY
## bbox:           xmin: 0.1 ymin: 51.5 xmax: 0.1 ymax: 51.5
## epsg (SRID):    4326
## proj4string:    +proj=longlat +datum=WGS84 +no_defs
##     name temperature       date         geometry
## 1 London          25 2017-06-21 POINT (0.1 51.5)

• Longitude and latitude identify locations on the Earth’s surface using two angles
  
  
• Angles depend on the choice of the Earth’s shape (sphere, ellipse)
  
  
• The shape is one part of a “coordinate reference system”: the datum

• Lat-Long coordinates aren’t “Cartesian” so Euclidean distances don’t work
  
  
• To put points onto a flat surface we use a projection
  
  
• Projections can’t perfectly reproduce all aspects of the Earth’s surface, because its not a perfect sphere or ellipsoid.
  
  
• Different projections preserve different properties, and are optimized for different regions of the Earth’s surface
  
  
• The projection is another part of the CRS

Let’s look at the CRS for the object we just created

st_crs(lnd_sf)

## Coordinate Reference System:
##   EPSG: 4326 
##   proj4string: "+proj=longlat +datum=WGS84 +no_defs"

CRS’s can be specified using + an epsg code + a proj4string definition

• The epsg code refers to one well-defined coordinate reference system
  
  
• For e.g., “4326” is standard lat-long
  
  
• proj4string has various options and gives more flexibility

Read in a lat-long dataset

x_ll <- read.csv("data/confirmed-sightings.csv")
x_ll <- st_as_sf(x_ll, coords = c("Longitude", "Latitude"), crs = 4326)
st_crs(x_ll)

## Coordinate Reference System:
##   EPSG: 4326 
##   proj4string: "+proj=longlat +datum=WGS84 +no_defs"

d_ll <- st_distance(x_ll[1, ], x_ll[2, ])

x_utm <- st_transform(x_ll, 
                      crs = "+proj=utm +zone=48 +datum=WGS84 +units=m +no_defs")
st_crs(x_utm)

## Coordinate Reference System:
## EPSG: 32648
## proj4string: "+proj=utm +zone=48 +datum=WGS84 +units=m
+no_defs"

d_utm <- st_distance(x_utm[1, ], x_utm[2, ])
c(d_ll, d_utm)

## Units: [m]
## [1] 229489.4 231902.2

occupancies <- st_read("data/Mongolia_SL.shp", quiet = TRUE)
head(occupancies)

## Simple feature collection with 6 features and 2 fields
## geometry type: POLYGON
## dimension: XY
## bbox: xmin: 26817.32 ymin: 5744980 xmax: 106817.3 ymax:
5804980
## epsg (SRID): 32648
## proj4string: +proj=utm +zone=48 +datum=WGS84 +units=m
+no_defs
## Team Occ geometry
## 1 A 0.20980194 POLYGON ((66817.32 5784980,...
## 2 A 0.28545357 POLYGON ((86817.32 5784980,...
## 3 A 0.25328456 POLYGON ((66817.32 5764980,...
## 4 A 0.14932811 POLYGON ((86817.32 5764980,...
## 5 A 0.28437589 POLYGON ((26817.32 5744980,...
## 6 A 0.09210202 POLYGON ((46817.32 5744980,...

plot(st_geometry(occupancies))

plot(occupancies["Team"])

plot(occupancies["Occ"])

occupancies %>% st_crs()  # check the CRS

## Coordinate Reference System:
## EPSG: 32648
## proj4string: "+proj=utm +zone=48 +datum=WGS84 +units=m
+no_defs"

occupancies <- st_read("data/Mongolia_SL.shp", quiet = TRUE) %>% 
    st_set_crs("+proj=utm +zone=48 +datum=WGS84 +units=m +no_defs")
occupancies %>% st_crs()

## Coordinate Reference System:
## EPSG: 32648
## proj4string: "+proj=utm +zone=48 +datum=WGS84 +units=m
+no_defs"

sights <- read.csv("data/confirmed-sightings.csv")
sights <- st_as_sf(sights, coords = c("Longitude", "Latitude"), 
    crs = 4326)
sights %>% st_crs()

## Coordinate Reference System:
##   EPSG: 4326 
##   proj4string: "+proj=longlat +datum=WGS84 +no_defs"

# reprojecting
my_crs <- "+proj=utm +zone=48 +datum=WGS84 +units=m +no_defs"
sights <-  st_transform(sights, crs = my_crs)

plot(sights["Elevation"])

raster_filepath <- system.file("raster/srtm.tif", package = "spDataLarge")
new_raster <- raster(raster_filepath)
plot(new_raster)

mydata <- expand.grid(x = 1:100, y = 1:100) %>% 
  mutate(z1 = x + y, z2 = x - y)
myraster <- rasterFromXYZ(mydata)
plot(myraster[["z2"]])

raster1 <- raster(nrows = 10, ncols = 10, res = 1, xmn = 0, xmx = 10, 
    ymn = 0, ymx = 10, vals = 1:100)
plot(raster1)

raster2 <- raster(nrows = 10, ncols = 10, res = 1, xmn = 0, xmx = 10, 
    ymn = 0, ymx = 10, vals = runif(100))
myrb <- brick(raster1, raster2)
plot(myrb)

dplyr verbs work with sf objects in intuitive ways

occ_gt90 <- occupancies %>% filter(Occ > 0.6)
plot(occ_gt90)

Some specialised spatial functions like st_intersects

sights_per_cell <- st_intersects(x = occupancies, y = sights)
sel_logical <- lengths(sights_per_cell) > 0
occ_with_sights <- occupancies[sel_logical, ]
plot(occ_with_sights["Occ"])

occupancies %>% group_by(Team) %>% summarize(meanOcc = mean(Occ)) %>% 
    head()

Can also summarize geometries

occ_per_team <- occupancies %>% group_by(Team) %>%
  summarize(meanOcc = mean(Occ),
            geometry = st_union(geometry))
plot(occ_per_team[1:12,"meanOcc"])

• similar syntax to ggplot2, tailored to maps
• shape is a spatial object with class sf, sp or raster.

map0 <- tm_shape(occupancies) + tm_fill()
map1 <- tm_shape(occupancies) + tm_borders()
tmap_arrange(map0, map1, nrow = 1)

map2 <- tm_shape(occupancies) + tm_polygons()
map3 <- tm_shape(occupancies) + tm_polygons("Occ")
tmap_arrange(map2, map3, nrow = 1)

• Multiple layers per shape

tm_shape(occupancies) + tm_polygons("Occ") + tm_text("Team", 
    size = 0.4)

• Multiple shapes

tm_shape(occupancies) + tm_polygons("Occ") + tm_shape(sights) + 
    tm_dots("Elevation", size = 0.5)

Color settings are especially important for maps. Here use of the same palette obscures detail.

• Sequential palettes are good for values that have natural ordering from low to high

tm_shape(occupancies) + tm_polygons("Occ") + tm_shape(sights) + 
    tm_dots("Elevation", size = 0.5, palette = "BuGn")

• Categorical palettes are best for data with no natural ordering

tm_shape(occupancies) + tm_polygons("Occ") + tm_shape(sights) + 
    tm_dots("Mountain", size = 0.5, palette = "Set3")

• Diverging palettes emphasise differences from some central reference point

tm_shape(occupancies) + tm_polygons("Occ") + tm_shape(sights) + 
    tm_dots("Elevation", size = 0.5, palette = "PRGn")

• faceting

tm_shape(occupancies) +
  tm_polygons(c("Occ", "Team")) +
  tm_facets(sync = TRUE, ncol = 2)

• geom_sf plots sf objects

ggplot() + geom_sf(data = occupancies)

• geom_sf plots sf objects

ggplot() + geom_sf(data = occupancies, aes(fill = Occ))

• Can plot multiple sf objects

ggplot() + geom_sf(data = occupancies, aes(fill = Occ)) +
  geom_sf(data = sights, colour = "red")

• scale_XXX_brewer for discrete scales, scale_XXX_distiller for continuous scales

ggplot() + geom_sf(data = occupancies, aes(fill = Occ)) +
  geom_sf(data = sights, aes(colour = Elevation)) +
  scale_colour_distiller(palette = "PRGn")

• provides an interface to the Leaflet JavaScript library
• leaflet() makes a map object that can be piped to other leaflet functions

leaflet() %>% addTiles()

• Set a starting origin and zoom level

leaflet() %>% addTiles() %>% setView(lng = 94.1, lat = 47.4, 
    zoom = 7)

• Set a starting origin and zoom level

leaflet() %>% addTiles() %>%
  setView(lng = 94.1, lat = 47.4, zoom = 4)

• Various base map options with addProviderTiles()

leaflet() %>% addProviderTiles("Esri.WorldStreetMap") %>%
  setView(lng = 94.1, lat = 47.4, zoom = 4)

• Various base map options with addProviderTiles()

leaflet() %>% addProviderTiles("Esri.WorldImagery") %>%
  setView(lng = 94.1, lat = 47.4, zoom = 4)

• sf objects can be passed as additional map layers
• must be in lat/long format

leaflet(data = st_transform(occupancies, crs = 4326)) %>%
  addProviderTiles("Esri.WorldImagery") %>%
  addPolygons()

• Multiple map layers

leaflet(data = st_transform(occupancies, crs = 4326)) %>%
  addProviderTiles("Esri.WorldImagery") %>%
  addPolygons() %>%
  addMarkers(data = st_transform(sights, crs = 4326))

pal <- colorNumeric("YlOrRd", domain = occupancies$Occ)
leaflet(st_transform(occupancies, crs = 4326)) %>%
  addProviderTiles("Esri.WorldImagery") %>%
  addMarkers(data = st_transform(sights, crs = 4326)) %>%
  addPolygons(color = ~pal(Occ), fillOpacity = 0.4) %>%
  addLegend(pal = pal, values = ~Occ) %>%
  addMiniMap(position = "bottomleft")

• Chapter 2 and Chapter 8 of Geocomputation with R
• sf vignette
• tmap vignette
• leaflet tutorial